The invention relates to a method of machining a hardmetal object by means of spark erosion in an aqueous solution, utilizing measures which counteract corrosion of said object. The invention also relates to a spark-erosion device comprising a work vessel and a sparking wire electrode which is led through said work vessel.
Using the spark-erosion technique, hardmetal objects having a complicated or a simple configuration can be rapidly and accurately manufactured. For this purpose, a spark-erosion device is used which comprises a metal sparking wire electrode. In the spark-erosion process, the wire electrode is usually kept at a negative, pulsed, electric voltage of maximally several hundreds of volts relative to the object to be machined. The wire electrode can be used to cut the hardmetal, because under the influence of electric discharges particles are removed from the hardmetal. Hardmetal objects are to be understood to mean, in particular, objects of sintered metallic carbides and borides which also complete up to several tens of percents by weight of metals as binders. A well known and advantageously used example thereof is sintered WC comprising 10% by weight of Co.
Spark erosion takes place under a liquid in a work vessel. At present, demineralized water is preferably used for this purpose. The liquid has a desired, high electric resistance of at least 5.10.sup.3 Ohm.cm. Organic compounds increasing the discharge stabilization and/or reducing the wear of the wire electrode, may be dissolved in the demineralized water. Spark erosion of hardmetal objects in aqueous solutions has disadvantages. From European Patent Specification No. 176.224, it is known that corrosion of the faces which are cut in the object in the spark-erosion process may occur. In EP 176.224, a special bipolar pulse frequency of the voltage on the sparking wire electrode is proposed to alleviate this corrosion problem. Applicants have established that this measure is insufficient. The use of a bipolar pulse frequency does not provide a complete protection against corrosion of the hardmetal object.